Project Summary The control of gene expression is at the core of biological development and homeostasis, and developmental pathways are often disrupted in disease processes. This is particularly true in hepatobiliary disease, as illustrated by the existence of over 20 congenital disorders associated with defects in the differentiation, morphogenesis, and maintenance of the bile ducts. This class includes biliary atresia, of which approximately 1/3rd of cases are associated with a developmental defect of the bile ducts (1, 2). In light of the fact that biliary atresia is the most common indication for pediatric liver transplantation, an understanding of the molecular basis of bile duct development may have a significant impact on human health. In recent years, an unexpected form of gene regulation has been discovered in which small RNA molecules known as microRNAs (miRNA) repress the expression of target genes through RNA interference (reviewed in (3-5)). There are over 500 human miRNAs and these may collectively regulate 20-30% of all genes (6-8). Virtually nothing is known regarding the function of miRNA in liver development and disease. To address this, we have performed the first large-scale study of miRNA expression during mouse liver development (see Preliminary Data), resulting in the identification of hepatic miRNAs whose spatio-temporal expression is suggestive of developmental functions. One of these (miR- 30a) is predominantly expressed in the ductal plate and bile ducts. We have utilized the zebrafish model system as a rapid, preliminary assay to test the function of miR-30a. As shown in the Preliminary Studies, zebrafish miR-30a is critical for the normal development and function of bile ducts. This proposal aims to investigate the biological and molecular function of miR-30a in the mammalian liver. In Aim 1, we will derive a mouse model of hepatic miR-30a deficiency and we will measure the effects of this deficiency on biliary structure and function. In Aim 2, we will use this model, a cell culture model, and computational prediction to perform a large-scale survey of miR-30a targets in the liver. The research proposed is significant because it may provide the first demonstration of a requirement for miRNA in liver development and it will significantly add to our limited understanding of the regulatory pathways controlling this biological process. It directly addresses goals of the NIH Action Plan for Liver Research regarding liver development (12). Project Narrative This proposal will study a recently-discovered form of gene regulation during the formation of the the [sic] bile ducts within the liver. This will help us to understand the normal development of the liver and the ways in which that process goes awry in a spectrum of diseases associated with malformations of the bile ducts. Those insights can then be applied towards better treatment of diseases of the liver and bile ducts.